The invention refers to a satellite cluster having at least two module satellites and a method for operating said satellite clusters.
Satellites in the conventional meaning are aerospace vehicles, which have a plurality of individual components, which are coordinated with each other, and are connected with each other to form a unit, to which for example the power supply, the state, position and temperature control, the telemetry, tracking and command subxe2x80x94system (TT and C) and the transponder with the antenna system all belong as the pay load. The transponder comprises, as a rule, a receiver part, a processing part (on-board processor) and a transmitter part, each with its own antenna. These individual components, to which more can be added depending on the special application, are combined in an aerospace vehicle, i.e. the satellite in the conventional sense.
The performance requirements for telecommunications satellites, for example with respect to their transmitting power, the number of transponders/spare transponders and the frequency bands have the final result that satellites in the conventional sense are very large and they become very heavy. These satellites can only be launched with the most powerful and most costly carrier rockets (for example Ariane, Proton etc.). The efforts which are made to reduce the size and the weight of the satellites with a view to the launch are constantly countered by the increasing requirements, which as a rule are connected with an increase in the structure and a rise in the weight of the satellites in the conventional sense.
Against this background the problem on which the invention is based is to create a satellite, which does justice to the constantly growing demands on performance and which is more flexible in use.
This problem is solved by a satellite cluster according to claim 1 and a method for operating said cluster according to claim 6.
According to one aspect of the invention, the modular satellite consists of at least one first module satellite, which has a device for the supply of power, for the control of the state, the position and of the temperature and for the control of the track of the module satellite, as well as a first pay load component and a first communications device to transmit data and/or signals to another module satellite, and a second module satellite which has a device for power supply, for control of the state, the position and the temperature and for control of the track of the module satellite, as well as a second pay load component and a second communications unit to transmit data and/or signals to another module satellite.
In accordance with another aspect of the invention, the modular satellite also comprises a third module satellite which has a device for the power supply, for the control of the state, the position and the temperature and to control the track of the module satellite, as well as a third pay load component and a third communications unit to transmit data and/or signals to another module satellite.
An important advantage in accordance with the invention is linked with the service life of modern telecommunications satellites. These modern telecommunications satellites have a service life of up to 15 years. But certain key technologies have a very much shorter life such as, for example, the signal processing on board the satellite (on board processing). Whereas in the case of conventional satellites the obsolescence of the on board processing means at once that this satellite can not be used or can only be used with limitations for modern duties, the modular satellite as in the invention provides the possibility that the module satellites on which the on board processing is realised can be withdrawn and can then be replaced by a new module satellite, which takes over the processing of the signals.
Furthermore, by providing the individual pay loads in the individual satellites it is achieved that the module satellite concerned has a clearly smaller launch weight than a satellite in the conventional sense, in which the pay load components which are distributed in accordance with the invention to a plurality of modules are combined. If one observes the total weight of all the module satellites which provide jointly the functions of a satellite in the conventional sense, it is certainly higher than the weight of the conventional satellite. The clearly reduced launch weight of the individual module satellites, however, brings with it so many advantages with respect to the carrier rockets which are needed for the launch that the rise in the total weight is neither technically nor economically a genuine drawback. In addition, requirements are conceivable which a conventional satellite could only meet with a size which can no longer be launched.
According to one aspect of the invention, the communications units build a transmission route by means of laser beams between the module satellites. To do this, the communications units have a laser transmitter and a laser receiver device.
Alternatively or in addition the communications units build up by means of microwaves a transmission route between the module satellites (1, 2, 3). The communication units have for this purpose a microwave transmitter and a microwave receiver device.
According to another aspect of the invention, the pay load component of a first module satellite is a receiver unit which has at least one receiver antenna in order to receive signals and/or data. The first module satellite can then receive the necessary control commands from the base station via the receiver antenna of the reception unit.
The pay load component of a second module satellite can be a signal processing device for the processing of signals and/or of data. The pay load of a third module satellite is then a transmitting antenna for sending signals and/or data via at least one transmitter antenna. The third module satellite can receive the necessary control commands from a base station (4) via the transmitting antenna of the transmitter device (11).
The three pay load components, i.e. the receiver device, the signal processing device and the transmitter device of the three module satellites which are equipped in accordance with the invention are only usable due to their interaction. The receiver unit of the first module satellite receives a signal which was sent out from a base station and transmits it to the signal processing device of the second module satellite, by which the processed signal is transmitted to the transmitter unit of the third module satellite.
The cluster satellites, which are collocated at the same geo-stationary orbital position, are controlled by one or more satellite ground control stations, which form integral part of the cluster satellite concept. Depending on the number of collocated cluster satellites, these satellite ground control stations manoeuvre the satellites safely and fuel efficiently by using one of the following collision risk strategies:
Longitude separation strategy
Eccentricity vector strategy
Inclination and eccentricity vector strategy
The collocation status of all cluster satellites must be continuously monitored via the satellite ground control stations by using highly accurate ranging and orbit determination tools such as trilateration ranging and interferometry.
In the following, the operating principles of each of these strategies are identified.
1. Longitude Separation Strategy
Cluster satellites are merely separated in longitude, therefore the number of modules which can be safely collocated is relatively small. Longitude separation works by ensuring that one dimension is separated at all times, namely longitude. In practice, however, the inclination vector differences will provide for additional separation most of the time, effectively creating latitude separation as well. The distance separation in longitude is ideally constant and can be expressed by the formula:
D=r sin (xcex94xcex).
North-south coupling, east-west manoeuvre dispersion and changing values of position uncertainty (sometimes seasonally depending) reduce the impressiveness of this method of collocating cluster satellites.
2. Eccentricity Vector Strategy
A radial and longitudinal separation is achieved by merely offsetting the eccentricity vectors of the orbit of each cluster satellite. More specifically, this strategy requires each cluster module to have different arguments of perigee.
The satellite ground control stations direct the eccentricity vectors of the cluster modules to different positions in inertial space while leaving their magnitudes the same. The satellite ground control stations ensure the eccentricity vector strategy by continually re-targeting these vectors to maintain the separation and do not allow transgression beyond a certain tolerance.
Unlike the longitude separation strategy, the distance between satellites will vary over the course of the orbit. There will be a relative phase difference in longitude liberation equal to the vector separation angle. The minimum distance will occur along the radial dimension and the maximum is in longitude. Since the orbits of the cluster modules are assumed to be in the same plane (though in practice they would probably not) there is no latitude separation.
3. Inclination, Eccentricity Separation Strategy
While the longitude separation depends on only one dimension to guarantee distance between satellites and the eccentricity vector strategy makes use of two dimensions, the inclination and eccentricity strategy employs all three dimensions. The satellite ground control stations collocate the cluster satellites by adding on top of the eccentricity strategy an inclination vector offset between all satellites, by this causing a separation in latitude.
In order to collocate cluster satellites based on the three strategies described above, the satellite ground control stations must apply a stringent and continuous monitoring of all cluster modules, positions. Practically spoken, this means that the distance separations and relative orientations for all cluster combinations must be projected forward in time to determine, if corrective manoeuvres are needed. This demands high accuracy for the post manoeuvre orbit. The satellite ground control stations must be able to perform corrective manoeuvres whenever required. This implicitly means that the satellite ground control stations have to have the means to determine the need, plan and implement such manoeuvres in short time. Furthermore, the satellite ground control stations must have the capability to execute two or more manoeuvres nearly simultaneously. The offline satellite ground control systems shall have the capability to compute distance separation and relative orientation of all collocated cluster satellites, which are important to calculate to estimate risk of collision and/or occultation. The ground systems shall be able to rapidly test the effects of small manoeuvre variations in the manoeuvre planning stage. The orbits of new cluster modules must be carefully phased to the collocation strategy. After the inter-satellite link has been established and is functional, traffic is switched and routed between the cluster modules.